1. The Field of the Invention
The present invention is directed to methods and compositions for cleaning silicon wafers in a two-phase liquid system with hydrofluoric acid. The system has two layers of immiscible liquids both of which contain hydrofluoric acid. Silicon wafers are cleaned by hydrofluoric acid in the top layer which is a nonpolar organic liquid. Metals from the silicon wafers and/or from the nonpolar organic liquid travel from the top layer to the bottom layer which is water. Treatment of silicon wafers in the inventive two-phase liquid system with hydrofluoric acid prevents metal and oxide deposits on the surfaces of the silicon wafers.
2. The Relevant Technology
In the microelectronics industry, methods for cleaning silicon wafers are continually being developed and optimized to meet the stringent demands for wafers having clean and smooth surfaces. As the device features continuously decrease to the deep sub-micron region, the product yield and device performance become even more dependent on the wafer cleaning technology.
A clean, chemically stable, and atomically uniform silicon surface is desirable prior to gate oxidation and silicon epitaxial growth in advanced ultra large-scale integration (ULSI) fabrication. It is well-known that metallic contamination on silicon surfaces can cause fatal effects on semiconductor devices. The metallic contamination on a silicon surface is preferably suppressed to less than 1E+10 atoms/cm.sup.2 in order to prevent defects.
Wet chemical processing, particularly hydrofluoric acid (HF) cleaning, continues to be a prevalent cleaning method in ULSI manufacturing despite difficulties in minimizing metallic contamination. Hydrofluoric acid is widely used as a wet etchant since silicon wafers are thereby obtained which have clean, chemically stable, and atomically uniform silicon surfaces. After aqueous hydrofluoric acid treatment, the surfaces of a silicon wafer are hydrogen-passivated bare silicon surfaces. Hydrofluoric acid treatment removes thermal and native oxides and is therefore an essential cleaning procedure and processing step of device fabrication.
During hydrofluoric acid wafer cleaning, metals including noble metals, have been found deposited on wafer surfaces by oxidation-reduction reactions resulting in severely deteriorated device performances. Some of the metals which are found deposited on silicon surfaces include copper, iron, calcium, potassium, magnesium, aluminum and nickel.
Metal deposition has been prevented by the addition of strong oxidizing agents such as ozone and hydrogen peroxide. Strong oxidizing agents such as ozone and hydrogen peroxide are useful for the prevention of metal deposition; however, the surface of the silicon wafers are also thereby reoxidized.
It has been postulated that metal deposition results when electrons are transferred from the silicon to the metal ion. Analysis of the deposition of copper (Cu), which has been a particular problem, provides an example of the mechanisms involved in metal deposition. The reaction in which a Cu.sup.2+ ion in a solution is metalized by taking electrons can be expressed by the following oxidation-reduction reaction equation: Cu.sup.2+ +2e-=Cu. The redox potential (E.degree.) of the metalization of a Cu.sup.2+ ion is 0.337 V. The reaction in which silicon in an aqueous solution releases electrons can be expressed by the following equation: SiO.sub.2 +4H.sup.+ +4e.sup.- =Si+2H .sub.2 O. The redox potential for the reaction of silicon in an aqueous solution is -0.857 V. A Cu.sup.2+ ion, which has a higher redox potential than silicon, takes electrons, is reduced to metallic copper, and is deposited onto a silicon surface. Silicon, which features a lower redox potential than the Cu.sup.2+ ion, releases electrons and is oxidized to become silicon dioxide (SiO.sub.2). The copper deposition onto a silicon surface in the solution is essentially induced by the oxidation-reduction reaction between silicon and copper ions. Pits found where copper particles deposit on the silicon surface in a diluted hydrofluoric acid solution provides evidence of this SiO.sub.2 formation.
A pit produced when silicon dioxide (SiO.sub.2) formed in the oxidation-reduction reaction is etched away by a diluted hydrofluoric acid solution is referred to as a Metal Induced Pit (MIP). The mechanism of metal deposition onto silicon surfaces in solutions begins with metal ions in the vicinity of a silicon surface withdrawing electrons from the silicon and becoming precipitated in a form of a metal such as metallic copper (Cu). It has been postulated that a nucleus of a metal particle is formed. When the metal nucleus adhering on the silicon surface features higher electronegativity than silicon, it attracts electrons from the silicon to become negatively charged. Other metal ions coming closer to the silicon surface gain electrons from the negatively-charged (electron-rich) metal nucleus and are precipitated around it. Accordingly, the metal nucleus grows into a larger particle on the silicon surface as more metal ions are precipitated. The silicon surface underneath the metal particles releases as many electrons as required by the metal ions to be charged, while SiO.sub.2 is thereby formed. In a diluted hydrofluoric acid solution, the formed SiO.sub.2 is etched away immediately and a MIP is made.
The metal nucleus is considered to be made where a silicon surface is electrically active. Electron exchange between metal ions and silicon is more likely to take place at kinks, steps, and areas where halide ions are adsorbed because these areas are more electrically active than the hydrogen-terminated areas on a silicon surface. The promotion of metal deposition by a trace level of halogen ions in hydrofluoric acid solutions can be explained by this mechanism.
A typical MIP formed from metallic impurities is about 0.1 .mu.m in diameter, which is also almost the same as a copper particle size. The depth of a typical MIP from peak to valley is about 8 nm. The MIP size can be fatal to device performance when it is considered that the thickness of a typical gate oxide is 8 to 15 nm.
One of the primary disadvantages of conventional wet chemical processing is the inability to eliminate the presence of metals from the wet etchant. Silicon wafers cleaned in a bath of wet etchant introduce metals into the bath. The concentration of metals within the bath increases as the bath cleans the silicon wafers. The bath must be eventually replaced when the metal concentration becomes too high.
The lifetime of a bath containing hydrofluoric acid is also relatively short when used to achieve a controlled etch. The concentration of hydrofluoric acid in a bath prepared for controlled etching is relatively low and must remain in a relatively narrow concentration range. The hydrofluoric acid is quickly consumed by the etching of the silicon wafers and must be replenished when the concentration approaches the lower limit of the narrow concentration range. There are no compositions or methods currently available which provide for the prevention of metal deposition and also the minimization of silicon surface reoxidation.
There are also no compositions or methods which extend the usefulness of hydrofluoric acid baths.